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heat, oxide, metal, silver, air, red and yellow

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LEAD, is a white metal, of a consider. ably blue tinge, very soft and flexible, not very tenacious, and consequently incapa• ble of being drawn into fine wire, though it is easily extended into thin plates un der the hammer. Its weight is very con. siderable, being rather 5reater than that of silver. Long before ignition, namely, at about the 540th degree of Fahrenhei0 thermometer, it melts ; and then begins to be oxyded, if respirable air be present. In a strong heat it boils, and emits fumes ; during which time, if exposed to the air, its proceeds with considerable rapidity. If melted lead be poured into a box previously rubbed with chalk, to prevent its action on the wood, and be continually agitated, it will concrete into separate grains, of considerable use in various mechanical operations, particu larly that of weighing. Lead is brittle at the time of congelation. In this state it may be broken to pieces with a hammer, and the crystallization of its internal parts will exhibit an arrangement in parallel lines.

This metal, during the progress of heat, first becomes converted into a dusky pow der, which by a continuation of the heat becomes white, yellow, and afterwards of a bright red, inclining to orange colour, called minium, or red lead. The process requires considerable management with regard to the heat and access of air, in the making of red lead. Many days are required for this purpose. If the heat be too great or rapid, the lead becomes con verted into a flaky substance, called litharge ; and a still greater heat converts it into a clear, transparent, yellow glass, which powerfully dissolves and corrodes metallic oxides or earths ; and on this ac count it usually finds its way through the crucibles in a short time. It acts more difficultly on argillaceous than on siliceous earths ; whence it is found that vessels made of clay mixed with broken pottery are preferable to those that are composed of clay and sand. The oxide of lead is a principal ingredient in most of the modern fine white glasses. It is more particularly calculated to form the dense glass, used to correct the aberration arising from colour in those telescopes which are known by the name of achromatic, because it com municates the property of separating the coloured rays from each other in greater angles than obtain in alkaline glasses at equal angles of mean refraction: The imperfection which most considerably af fects this kind of glass is, that its density is seldom uniform throughout. The ir

regularities show themselves in the forms of veins, which greatly disturb the regu lar refraction.

Lead is not much altered by exposure to air or water, though the brightness of its surface, when cut or scraped, very soon goes off It is probable that a thin stratum of oxide is formed on the surface, which defends the rest of the metal from corro sion.

All the oxides of lead are very easily re duced. Minium, when exposed to a strong heat, gives out part of the oxygen it ab sorbed during its oxidation ; but, like the other oxides of this metal, it requires the addition of some combustible substance for its complete revival : a familiar in stance of this revival is seen by exposing the common wafers to the flame of a can dle. The wafers are coloured with minium, which is revived by the heat and inflammable substance of the wafer, so that it falls down in metallic globules.

Lead is found native, though seldom ; and also in the form of an oxide, called native ceruse, or lead ochre, or lead spar, of various colours, red, brown, yellow, green, bluish, and black. These ores, when freed as much as possible from earthy matter, may be dissolved in diluted nitrous acid. Oxide of iron is usually thrown down from the solution by boiling. If the lead he then precipitated by the carbonate of soda, and weighed, 132 grains of the dry precipitate will correspond with 100 grains of lead in the metallic state. If the precipitate be suspected to contain copper, it may be separated by digesting in ammonia. If it be supposed to contain silver and copper, the precipi tate may again be dissolved in nitric acid, and separated by the addition of muriatic acid ; which, combining with the metal, produces the muriates of silver and of lead ; the latter of which, being soluble in thirty times its weight of boiling water, may be washed off, while the silver re mains undissolved ; or the silver, if alone in the precipitate, may be taken up by ammonia, which will leave the oxide of lead of the same value, with regard to weight, as the foregoing.

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